System for electroless plating of copper and composition

ABSTRACT

ng agent for cupric ion, a reducing agent, an alkaline compound for adjusting pH, the improvement comprising the addition of a stabilizer consisting of an organic thiol compound selected from the group of organic acids having the characteristics of chemical tautomerism in the carboxylic group selected from the group consisting of dithiolterephthalic acid, thiolbenzoic acid or thiolacetic acid.

United States Patent 1191 Kishita 14 1 Sept. 2, 1975 SYSTEM FOR ELECTROLESS PLATING OF COPPER AND COMPOSITION 21 Appl. No.: 389,119

[52] US. Cl. 106/1; 117/130 E [51] Int. Cl. C23C 3/02 [58] Field of Search 106/1; 117/130 E [56] References Cited UNITED STATES PATENTS 3,257,215 6/1966 Schneble 106/1 3,485,643 12/1969 Zeblisky 106/1 Primary Examiner-Theodorc Morris Attorney, Agent, or Firm-15. Lawrence Brevik ABSTRACT The electroless plating of copper is dependent upon incipient instability of a lightly sequestered copper ion in the presence of a reducing agent; the stability of the solution is greatly improved by incorporating therein a small amount of tautomeric acid having the formula 1- 9 R-C-SH:R-C=S 2 Claims, No Drawings SYSTEM FOR ELECTROLESS PLATING OF COPPER AND COMPOSITION BACKGROUND OF THE INVENTION The electroless deposition of copper from a solution of copper sulfate or one of its other soluble salts upon a surface of a metal higher in the electromotive series of metals is one of the simplest of experiments to a novice in chemistry. In the past twenty years or more, the application of copper to surfaces other than a metal higher in the electromotive series of metals has developed into a highly desirable, technically feasible process.

The electroless or autocatalytic deposition of copper upon metallic and non-metallic substrates has been extensively researched as evidenced by the broad scope of patent coverage; but all data indicates several basic requirements which consist of the following:

a. A water-soluble copper salt b. A complexing agent for copper c. A reducing agent for the copper ion d. A compound for adjusting the pH of the solution e. Water Although literature on the complexing of copper in basic solution preceeded it, one of the first patents in the field employing all these necessary ingredients was that of Allen E. Cahill, et. al., Auto Catalytic Copper Plating Process and Solution, U.S. Pat. No. 2,874,072 issued on Feb. 17, 1959. Since that date numerous other patents have issued both in the U.S. and abroad, embellishing the composition of Cahill.

As the very nature of an autoeatalytic deposition is a reaction which takes place without the application of exterior energy, it is of paramount importance that the reaction take place only under very carefully controlled conditions to eliminate undesirable decomposition of the plating bath. It is quite obvious, therefore, that a process of this type must be a result of a number of compromises. In the first instance, the redox potential of the reductant and the metal ion must be balanced to insure reduction of the metal ion at a very carefully controlled rate. Second, the redox potential is affected by the pH of the solution. Third, the complexing agent must be of such a nature as to release the metal ion in the proper amounts and at the proper time; the characteristics of the complexing agent are materially affected by the pH of the solution as well as the redox potential of the reducing agent and the metallic It is, therefore, apparent that with the complex nature of a controlled autocatalytic reaction of this type, the principal development would lie in the field of stability as related to rate and quality of plating. Among the most prominent developments as evidenced by patent disclosure are the following:

Cyanide radical UAS. 3,095.30; US. 3,485,643 U.S. 3,403,035 Polyalkylenc oxides US. 3,607,317 Colloidal cellulosic cthers U.S. 3,329,51 Alkine alcohols U.S. 3,457,089 Phenanthroline 8: iodide ions U.S. 3,615,736 Oxygen U.S. 2,996,408 Selenium U.S. 3,492,135 Sulfur compounds U.S. 3,361,580 US. 3.257.215 U.S. 3.425.416 U.S.'3.392,035 U.S. 3.453.123,

Although all listed patents contain information on improving the stability of the copper plating solutions, this extensive list is enumerated to show, in part, the plating composition. The invention herein, however, is concerned only with the latter group of developments, ie., those associated with the use of sulfur compounds to stabilize the electroless copper solution.

The first patent issued containing sulfur compounds as a stabilizer is that of Schneble, et al., U.S. Pat. No. 3,257,215, Electroless Copper Plating, issued June 21, 1966 in which Schneble found that a very broad range of sulfur-containing compounds exhibited an effect upon the stability of the electroless copper plating bath. Although disclosure of specific sulfur-bearing compounds, both organic and inorganic, as individual species as well as broad families, one class of sulfurbearing organic chemicals has had no reference in the prior art; this is the thiol carboxylic acids. These acids, being tautomeric, present an instability in solution which produces a synergistic effect which far exceeds the stabilizing characteristics of the sulfur compound used in the prior art and provides a new composition employing a tautomeric compound which contains sulfur, not merely an extension of the prior art.

SUMMARY OF THE INVENTION In the electroless plating of copper metal on nonmetallic surfaces, the surface is first cleaned to remove all traces of oily substances and other contaminating materials. Thereafter the surface is treated in a stannous chloride palladium chloride sensitizing solution which creates a surface with macro-molecular metallic deposits which are sites for deposition of copper in the subsequent operation. The sensitization step is well known in the art and will not be further discussed here, except to state that after the surface is sensitized, it must be thoroughly rinsed in distilled or deionized water to prevent contamination of the copper bath for it is quite obvious that if the tin and palladium are plated out upon the surface of the non-metallic material that they would be readily reduced by the reducing agent in the copper bath and thereby produce sites for precipitation of the copper within the copper plating solution, which would destroy the effectiveness of the electroless copper bath. This chain of events actually happens in practice and contamination control is very rigid in a production line.

Therefore, after sensitization, the non-metallic is rinsed thoroughly and then treated in the electroless copper solution.

The prior art has taught that an electroless copper plating solution can be composed of the following:

. A copper salt (cupric) Formaldehyde A complexing agent for cupric ion A pH adjusting compound A brightener A stabilizer Following are specific examples successfully employed in current production:

Cupric sulfate 12 grams/litre Coppcr sulfate I2 g/l Formaldehyde (3771) I8 ml/l Rochelle salts 30 g/l NaOH 10 g/l Thioglycolic acid (70%) .()()l ml/l Water to one litrc In subsequent discussions, this composition shall be known as the thioglycolic acid plating solution.

The composition which I employ consists of acid organic compounds having the following composition:

wherein R is an alkyl or aryl radical; all the acids which I claim contain the SH radical are called thiol acids. These acids are all tautomeric in structure, ie., they tend to exist in the following equilibrium:

R- -SH R-C-SH:ZR-C=S This characteristic is undoubtedly a significant factor in their unique quality to stabilize an electroless copper plating bath and it creates a new and highly successful product never previously reported.

As previous investigations indicate that sulfur compounds have been employed in electroless copper plating solution solely as stabilizers, a series of tests were undertaken to compare stability of tautomeric thiol acids to that of the MBT and thioglycolic acid formulations. These results are tabulated below:

STABILITY AT 70F TIME STABILIZER TO INITIAL TIME TO PRECIPITATION DECOM POSI- TION MBT Cyanide l2 hours l8 hours Thioglycolic Acid 120 hours I62 hours Thiolacelic Acid I8 hours 25 hours Thiolbcnzoic Acid Zl hours 29 hours Dithiolterephthalic Acid 163 hours 2l2 hours No Stabilizer 30 seconds 30 seconds All stabilizers were used at a concentration of 1 ppm by weight; the MBT was employed at a level of 1 ppm and the cyanide is sodium cyanide at a concentration of mg/l. The basic bath to which the stabilizers were added was the following:

Cupric sulfate 12 g/l Formaldehyde (3771 aqueous) l8 ml/l Rochelle Salts 30 g/l NuOh 10 g/l Watcr to one litre To facilitate the decomposition with a basic approach which would be characteristic of actual operation conditions, a single drop (.04 ml) of tin palladium activator was added to a ml sample of each of the solutions listed above, immediately after the solution was prepared. This created a standard which was reproducible within about 5% accuracy as was indicated by several repetitive tests.

As one drop from a pipette which was calibrated at 25 drops per ml would introduce 40 ppm of the tin palladium catalyst into the 100 ml specimen, this was characteristic of an actual plating solution during the plating cycle.

The data indicates the usefulness of the thiol acids as a stabilizer comparable to the conventional industrially useful MBT/Cyanide composition. The most successful composition from a stability standpoint was the Dithiolterephthalic Acid composition.

However, in addition to the stability, another significant characteristic of the electroless copper plating solution is the appearance of the plated surface. In actual practice the electroless copper is plated upon either a non-metallic surface such as a plastic or ceramic; or it is plated upon a metallicsurface such as a copper or stainless steel surface. In general, when the plating is upon a non-metallic surface, the surface must first be activated; this is generally accomplished by dipping the cleaned, non-metallic surface in a tin palladium catalyst previously discussed. The surface characteristic of the copper plated from the electroless copper solution is quite different when plated upon non-metallic than that plated upon the metallic surface, but in both instances it is desirable to end with a copper plating which is bright, for this indicates a clean copper plate; a dark plating usually implies that the copper surface after plating contains impurities which may result in discontinuities in the surface; inclusions of chemicals in the plated copper; or a powdery deposit. In all these instances, failure of the plating could result which would defeat the purpose for which the plating is applied.

It is quite obvious, therefore, that brightness of the plated surface is an excellent indication of the quality of the coating. Until the discovery of the use of cyanide as an additive to the electroless bath, the electroless plating process was in jeopardy as an industrial process; since that time however the process has found increasing application.

The characteristic of the tautomeric thiol acids as additives has not only improved the stability of the plating solution, but has also produced a bright finish, comparable to the cyanide solutions, but without the use of the exceedingly toxic cyanide ion. As the use of cyanide, from a disposal standpoint is becoming more critical, the significance of my invention becomes more important.

The following data was gathered to verify the metallic surface characteristics of the various activators. The MET/Cyanide solution previously disclosed was used as a standard. When the surface herebelow is described as bright, the metallic lustre is comparable to that produced by the MBT/Cyanide solution when freshly prepared.

PLATING ON NON-METALLIC (EPOXY-FIBREGLASS) 70F ACTIVATOR CHARACTERISTICS Thioglycolic Acid Dark Thiolacetic Acid Bright Thiolbenzoic Acid Bright Dithioltercphthalic Acid Bright PLATING ON COPPER SURFACE 70F ACTIVATOR CHARACTERISTICS FRESH 48 Thioglycolic Acid SOLUTION HOURS OUIESCENT As the research continued, it became obvious that the tautomeric thiol acids were very useful compounds for stabilizing electroless copper solutions, but one compound stood out, ie., dithiolterephthalic acid. It was therefore undertaken to further investigate this compound in relationship to the most widely used proprietory plating solution, Shipley Chemical Companys CP-7O which is recommended for use as a rapid plating solution at 120F. In my evaluation CP-7O was found to plate upon an activated epoxy-fibreglass surface at a rate of 0.25 milli-inches per hour; my compound employing 1.0 ppm of dithiolterephthalic acid stabilizer was found to plate at a rate of 0.30 milli-inches per hour at a temperature of 1 F. At room temperature, the same composition was found to plate at a rate of O. I 0 milli-inches per hour at 70F which is about twice the rate of MBT/Cyanide bath.

A still further characteristic which is desirable is the ductility of the copper which is plate. Ductility is a characteristic of the purity of the copper deposit and the crystal structure. A brittle plate may crack on flexing and flake off the substrate. It was found that when a film of copper plated from the dithiolterephthalic acid was removed from the substrate, a film 0.6 milliinches thick could be folded back upon itself with finger pressure to cause a permanent crease in the copper film and that the film could be flexed and unfolded in this manner 13 times before failure as evidenced by cracking at the bend line; likewise, a 0.3 milli-inch film was flexed for 34 cycles. The ductility of this plating exceeded all other plating solutions tested.

All the thiol acids were tested over a range of 0.01 ppm to 50 ppm. In ranges below 0.1 ppm, the stabilizer was not useful; above 5 ppm, the stabilizer was so effective as to stop the plating. The highly selective range of 0.1 5 ppm of thiol stabilizer was the most effective range for all thiol acids tested. It should be pointed out that there are a great number of thiol acids known but they are generally scientific curiosities rather than commercially available chemicals. For this reason, the tests reported above were made on those thiol acids which were available, and even these were available only in laboratory quantities.

The compounds tested were all outstanding stabilizers. This unique characteristic is believed to be due to their tautomeric structure, as was previously stated.

It is the general theory of electroless plating of copper from an alkaline solution that the copper must be present in a complex form, that is, the copper complex must be alkaline soluble or all the copper would immediately precipitate as Cu(OH) However, to produce a copper plate, it is essential the the Cu ion be reduced to Cu. In its simplest state the equation is represented as follows:

Cu 2e Cu In fact, the Cu is complexed but the complexing agent must be of such a nature as to provide a number of free Cu ions to enter into the reduction reaction, ie.,

Cu Complex 2 Cu Complexing Agent Generally, the complexing agent is tartaric acid or ethylene diamine tetra acetic acid but others may be used. As this equilibrium is essential to provide the necessary cupric ions for reduction, and as the cupric ions are removed from the solution by reduction, the equilibrium shifts to the left.

It is generally believed that in the reduction of cupric ions to copper, the following reaction takes place:

Equation (1) is the preferred reaction; however,

there is a question of whether this reaction is instantaneous or whether the reaction (3) is the actual reaction which produces the copper plate.

It is known, however, that of the various reactions that take place, some Cu is produced which reacts readily with the OH present to produce insoluble Cu(OH) as represented by the reaction (4). As

Cu(OH) is very insoluble and no chelating or complexing agent is present to react with the Cu when it is formed, the Cu(OH) precipitation takes place. This is a highly undesirable phenomenon as any precipitate in the copper plating solution provides sites for the plating of copper, thereby autocatalytically reducing the copper in the solution and depleting the solution without economic benefit.

It is, therefore, a requirement of an acceptable electroless copper plating solution that a complexing agent or agent capable of preventing the formation of Cu ion in the solution be present.

It has been found that sulfur compounds are capable of forming'stable chelates with Cu to render it innocuous in solution. It is further found that only small amounts of sulfur-bearing compound will perform this function, ie., 0.1 50 ppm of sulfur-bearing compound, but in all instances, sulfur compounds present in amounts above this value will stop the plating action entirely, indicating that formula (3) may be the predominate reaction in the plating of copper from an electroless bath. in general, sulfur compounds produce a dark coating which would indicate the presence of impurities in the plating which are highly undesirable. To eliminate this undesirable characteristic, it has been found necessary to add another complexing ion, ie., CN to produce the desirable bright metal plating. With the addition of both cyanide and sulfur compounds, it is difficult to control the bath as each are present in such small quantities that it is almost impossible to provide practical analytical methods for control purposes. It is the gist of my invention that this control problem can be eliminated by the use of asingle compound which creates its own equilibrium according to the well known chemical dissociation principles of equilibrium. That is, when a tautomeric compound is placed in solution. the dissociation equilibrium is easily controlled by factors more readily analyzed, such as pH. temperature and copper concentration, and that shifting any one of these factors will provide a predictable shift in the equilibrium. As the temperature tends to exist in stable equilibrium under a specific set of conditions, there is a shift in the equilibrium to effect a shift in conditions of the bath which affects the equilibrium. This effect is characterized by the following example. It is believed that the thiol acid ion and as the Cu ion is produced, it effectively combines with the thiol structure, thereby decreasing the concentration of thiol acid from the solution and the equilibrium shifts as follows:

that is, the equilibrium of the two tautomeric compounds requires that as the -SH compound is removed, the =S compound must be reduced also, which causes a shift to the SH from the =S compound.

it is to be understood that this theory of the effectiveness of my invention is disclosed solely for the purpose of explaining what I believe to be the logical, scientific analysis of the use of the thiol acids and in no circumstances is this explanation meant to limit or expand the claim appended hereto.

I claim:

1. An electroless copper plating solution which comprises a water soluble copper salt, a complexing agent for cupric ion, a reducing agent, an alkaline compound for adjusting pH, the improvement comprising the addition of a stabilizer consisting of an organic thiol compound selected from the group of organic acids having the characteristics of chemical tautomerism in the carboxylie group selected from the group consisting of dithiolterephthalic acid, thiolbenzoic acid or thiolacetic acid.

2. The composition of claim 1, in which the stabilizer is present in the range of 0.10 to 5.0 parts per million based upon the weight of the plating solution. 

1. AN ELECTROLESS COPPER PLANTING SOLUTION WHICH COMPRISES A WATER SOLUBLE COPPER SALT, A COMPLEXING AGENT FOR CUPRIC ION, A REDUCING AGENT, AN ALKALINE COMPOUND FOR ADJUSTING PH, THE IMPROVING COMPRISING THE ADDITION OF A STABLIZER CONSISTING OF AN ORGANIC THIOL COMPOUND SELECTED FROM THE GROUP OF ORGANIC ACIDS HAVING THE CHARACTERISTICS OF CHEMICAL TAUTOMERISM IN THE CARBOXYLIC GROUP SELECTED FROM THE GROUP CONSISTING OF DITHIOLTEREPHTALIC ACID, THIOLBENZOIC ACID OR THIOLACETIC ACID.
 2. The composition of claim 1, in which the stabilizer is present in the range of 0.10 to 5.0 parts per million based upon the weight of the plating solution. 